Synchrotron radiation is short for synchrotron accelerator radiation, which is high-intensity and highly-collimated light beams emitted when high-energy electrons deflect in a magnetic field. In order to generate synchrotron radiation having a higher intensity, a large number of present synchrotron radiation devices use undulators. The undulator generates a magnetic field which varies periodically. High-energy electron beams periodically moves in the undulator, and the generated light has a higher intensity due to an interference effect. With the development of accelerator technology, the divergence of electron beam current becomes smaller and smaller, and the thermal loads (the sum of the power of all energy photons) on optical elements (such as mirrors, gratings and crystals) becomes larger and larger. On the other hand, with the improvement of processing technology, the surface machining errors of optical elements can fully meet the requirements, and the surface machining errors of optical elements (such as deformation) caused by thermal loads have become a decisive factor which affects the performance of light beam line. Therefore, high thermal loads have become an urgent problem to be solved by modern synchronous radiator devices. As for synchrotron radiation, the heat loads are emitted, due to a relativistic effect, at a very small divergence angle (defined as a divergence angle of 90% photons, and 0.008° for electron energy of 3.5 GeV) along a velocity direction of electron movement. As for undulators which generate circularly polarized light, the velocity direction thereof is never along the axis direction of the undulators due to the helical movement of electrons. The direction of extreme value of the thermal load deviates from the axis of the undulator. Most of the thermal loads can be filtered out by a diaphragm, and will not be irradiated onto optical elements. As for linearly polarized undulators as commonly used, electrons make a serpentine movement in a horizontal plane or in a vertical plane, and the velocity direction thereof will sweep through an axis of the undulator, thereby causing a larger thermal load of the light beam line.
In order to solve the problem of high thermal load, Japanese Dr. Tanaka proposed a Figure-8 undulator structure (T. Tanaka and H. Kitamura, nuclear instruments and methods in physics research, section A 364 (1995), 368-373): linearly polarized light can be generated by using left and right lateral movements of electronics cascade and the coherence of circularly polarized light, the ratio of the magnet period in a horizontal direction to a vertical direction is 1:2, and the movement trajectory of electrons is shown in FIG. 1. Because the movement trajectory of electrons is a left and right lateral movement, the velocity direction of electrons will never be along the axis of undulator, such that the thermal loads deviate from the axis of the undulator. At the same time, the coherent light is strongest along the axis of the undulator, thus the problem of thermal loads generated when synchrotron radiation generates linearly polarized light is solved. However, the Figure-8 undulator merely generates linearly polarized light and does not generate circularly polarized light. Since the second harmonic of the Figure-8 undulator along a long period direction is capable of interfering with the fundamental wave along a short period direction, pure linearly polarized light cannot be generated. The APPLE (Advanced Planar Polarized Light Emitter) undulator proposed by professor Sasaki (Sasaki, nuclear instruments and methods in physics research, section A 347 (1994), 83-86) can generate synchrotron radiation polarized light having arbitrarily polarizations by a relative displacement between the moving magnet groups and the stationary magnet groups, and the arrangement structure of magnet is shown in FIG. 2. However, when linear polarized synchrotron radiation is generated, the magnetic field of APPLE undulator is the same as that of a linearly polarized undulator as commonly used, and therefore the problem of thermal loads cannot be solved. Later, professor Sasaki proposed another APPLE-8 undulator (S. Sasaki et. al., EPAC98, p 2237 (1998)) based on the APPLE undulator and the Figure-8 undulator. The undulator consists of two standard APPLE magnet groups. An APPLE undulator consisting of four rows of inner magnet groups is used to generate synchrotron radiation. An APPLE undulator consisting of outer four rows of magnet groups cooperates with the inner APPLE undulator to create figure-8 movement. The inner undulator and outer undulator have a period ratio of 1:2. As shown in FIG. 3, synchrotron radiation polarized light having arbitrary polarization can be generated through the displacement of diagonal four-column moving magnet. However, since the Figure-8 undulator cannot generate pure linear polarized light, the degree of linear polarization of the APPLE-8 undulator can only reach 82%.
In order to generate arbitrary polarization synchrotron radiation of low thermal loads, the present inventors proposed a running mode of a Knot (knot type) undulator based on an electromagnetic undulator (S. Qiao et. al., Review of Scientific Instruments 80 (2009), 085108), which thoroughly solves the problem of thermal loads of synchrotron radiation. The Knot undulator generates linearly polarized synchrotron radiation having low thermal loads by using left and right lateral movements of electronics cascade. Since the ratio of a magnet period in a horizontal direction to a vertical direction is 3:2, the degree of linear polarization is as high as 99.2%. Moreover, left and right laterally circularly polarized light can be generated through the switching of electromagnet polarity and current. However, due to the hysteresis effect of electromagnets, the size of magnetic fields is related to the history of magnetizing current, which may adversely affect the stable operation of accelerators. In addition, the electromagnet needs to be electrified to maintain its magnetic field, which is unfavorable to energy saving and emission reduction. Considering the above two points, based on the structure of the Knot undulator proposed by the present inventor, professor Sasaki proposed a APPLE-Knot undulator structure as shown in FIG. 4 based on permanent magnet. The APPLE-Knot undulator structure is composed of inner four rows of standard APPLE magnet groups and four rows of APPLE magnet groups having a vacant region outside. Due to the introduction of vacant region, the period ratio of the magnetic field generated by the outer magnet groups and the magnetic field generated by the inner magnet groups is 3:2. In this configuration, the magnetic fields of the middle four rows of magnets provide the magnetic field required by the synchrotron radiation. In the following descriptions, it is named as a main magnetic field or an APPLE magnetic field. The magnetic field of the outer four rows of magnets has a cascading 90° and −90° phase difference with the main magnetic fields, such that a Knot movement mode is generated. In the following descriptions, it is named as an auxiliary magnetic field or a Knot magnetic field. FIG. 5 shows the magnetization directions of each permanent magnet units corresponding to the main magnetic field and the auxiliary magnetic field in each row of the permanent magnet structure in FIG. 4. However, if the structure as shown in FIG. 4 is used, since the distance between the outer four rows of magnets is large, the Knot magnetic field intensity generated by the outer four rows of magnets is too weak, and the velocity direction of electrons deviates from the axis of the undulator by a limited angle, thus the peak direction of the heat loads deviates from the axial of the undulator by a limited angle, and most of the thermal loads cannot be effectively removed.